Multi-leaf collimators are comprised of a plurality of individual parts (known as “leaves”) that are formed of a high atomic numbered material (such as tungsten) that can move independently in and out of the path of the radiation-therapy beam in order to selectively block (and hence shape) the beam. Typically the leaves of a multi-leaf collimator are organized in pairs that are aligned collinearly with respect to one another and that can selectively move towards and away from one another via controlled motors. A typical multi-leaf collimator has many such pairs of leaves, often upwards of twenty, fifty, or even one hundred such pairs.
In many application settings the aperture(s) formed by such a multi-leaf collimator selectively changes during the course of a single radiation treatment session for a given patient (to accommodate, for example, exposing the treatment target to radiation from a variety of different angles (or fields) as occurs during so-called arc therapy).
The sliding window sequencing method is a typical prior art approach to using a multi-leaf collimator that entails creating a leaf-motion pattern where leaf pairs start from a closed or open position on one side of the fluence map and then travel across the fluence. The gap between the leaf tips in a single leaf-pair is modulated during the motion so that every location along the leaf trajectories is exposed to radiation for a length of time that corresponds to the optimal fluence map pixel value at the same location. The sliding window sequencing method is beneficial in that an arbitrary optimal fluence map can often be transformed into leaf sequences with high fidelity and the sequence can be created without imposing any beam holds.
The sliding window technique also has limitations and shortcomings, however. For example, the produced leaf motion patterns are often not very robust for intra-fraction motion during the treatment session. This shortcoming is usually monitored by checking the average leaf-pair opening of the leaf sequence and if necessary the optimal fluence can be re-optimized with criteria that produce smoother fluences. As another example, the sliding window method works best when any leaf can create modulation without restrictions from neighboring leaves. Limiting constraints may occur, however, if the multi-leaf collimator design has leaves in two layers or if neighboring leaves cannot interdigitate. Such restrictions can usually be solved by further modifying the sliding window pattern, but this solution often increases the necessary monitor units and further reduces the average leaf-pair opening.
Another leaf sequencing method, sometimes referred to as the multiple static segments process, addresses at least some of the foregoing concerns but unfortunately this approach cannot reliably reproduce the optimal fluence with the same fidelity as the sliding window approach and the multiple static segments process also typically requires multiple beam holds that can, in turn, require an unacceptable amount of time for the treatment machine and/or the session parameters of the treatment session.
Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present teachings. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present teachings. Certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. The terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.